Digestion of wood with oxygen in the presence of alkali

ABSTRACT

A process is provided for the production of cellulose of high brightness from wood by digestion with alkali and oxygen in aqueous solution under moderate oxygen pressure, limiting the amount of alkali at the start of the digestion to less than that required, and progressively adding alkali as the digestion continues, while maintaining the digestion liquor at a pH within the range from about 9.2 to about 13.

United States Patent [191 Samuelson et al.

[ *Oct. 30, 1973 DIGESTION OF WOOD WITH OXYGEN IN THE PRESENCE OF ALKALI Inventors: Hans Olol Samuelson; Goteborg Sture Erik Olof Noreus, both of Ornskoldsvik, Sweden Assignee: Mo Och Domsjo Aktiebolag,

Ornskoldsvik, Sweden Notice: The portion of the term of this patent subsequent to Mar. 28, 1989, has been disclaimed.

Filed: Feb. 26, 1971 Appl. No.: 1 19,375

Related US. Application Data Continuation-in-part of Ser. No. 869,875, Oct. 27, 1969, Pat. No. 3,652,385, and a continuation-in-part of Ser. No. 36,670, May 12, 1970, Pat. No. 3,652,386.

Foreign Application Priority Data May 13, 1970 Sweden 6569/70 US. Cl 162/65, 8/111, 162/76,

162/86 Int. Cl. D2lc 9/10 Field of Search 162/65, 84, 72, 76,

References Cited UNITED STATES PATENTS 3/1972 Noreus et a1. 162/65 X OTHER PUBLICATIONS Rydholm, Pulping Processes, pgs. 1044-1045.

Primary Examiner-S. Leon Bashore Assistant ExaminerArthur L. Corbin Attorney-Janes & Chapman [57] ABSTRACT A process is provided for the production of cellulose of high brightness from wood by digestion with alkali and oxygen in aqueous solution under moderate oxygen pressure, limiting the amount of alkali at the start of the digestion to less than that required, and progressively adding alkali as the digestion continues, while maintaining the digestion liquor at a pH within the range from about 9.2 to about 13.

36 Claims, No Drawings DIGESTION OF WOOD WITH OXYGEN IN THE PRESENCE OF ALKALI This application is a continuation-in-part of Ser. No. 869,875, filed Oct. 27, 1969, now U.S. Pat. No. 3,652,385, and Ser. No. 36,670, filed May 12, 1970, now U.S. Pat. No. 3,652,386.

Most cellulose pulp produced today in commerce is prepared by the sulfate or kraft process, in which wood is digested or pulped with alkali and sodium sulfide, and sodium sulfate is used as the make-up chemical to cover losses in the recovery cycle. The greater part of the remainder of the cellulose pulp is produced by the three variants of the sulfite process, in which the active digestion or pulping chemicals comprise acid sulfite, bisulfite, or neutral sulfite. Some pulp is still produced by the so-called soda process, in which sodium carbonate is used instead of sodium sulfate as the make-up chemical, but this process has a number of serious disadvantages, compared to the kraft process, particularly a low pulp yield and a poor pulp quality, and consequently this process is no longer used to any considerable extent. However, the sulfate process also has a number of disadvantages, the most serious one, from the standpoint of pollution of the environment, being the discharge of sulfur dioxide, hydrogen sulfide and other waste gases, as well as the black liquor effluent, which must be captured, processed for recovery, and recycled to maintain an economical operation. Even with maximum recovery and reutilization of waste chemicals, contamination of the atmosphere and of waterways adjacent a sulfate pulp mill remains a serious problem which has not been entirely overcome, and as a result, many kraft pulp mills today are faced with the necessity of developing a workable and practical alternative to the sulfate process. This alternative is not provided by the sulfite or soda processes.

It has been suggested that the part played by the sulfide in freeing the cellulose pulp fibers from the wood can be taken by oxygen.

Harris U.S. Pat. No. 2,673,148, dated Mar. 23, 1954, proposed an oxygen digestion process using quite high oxygen pressures, of the order of at least 800 psi. This was thought necessary in order to obtain and maintain a sufficiently high oxygen concentration in the digestion liquors. This is one of the serious problems in oxygen digestion processes due to the fact that oxygen is a gas and not capable of being solubilized in the digestion liquor by the expedients employed up to now. However, the results obtained in this process were not satisfactory.

Grangaard and Saunders, U.S. Pat. No. 2,926,114, dated Feb. 23, 1960, stated that oxygen prior to 1957 had been used both at low and at high oxygen pressures. However, at low pressures, the pulping was inadequate, and the process had to be used only as a single stage in a multiple stage pulping process, using more conventional pulping chemicals to complete the pulping. At the high pressures, the pressures are so high, large volume batch digesters cannot be readily constructed to withstand them. Grangaard et al. proposed a digestion at pH 7 to 9 over at least a major portion of the cooking time, ranging up to 9.4 at the end of the cook, under oxygen pressures of 40 to 250 psi, using conventional batch digesters. The pH is maintained within the desired range by a buffer such as sodium bicarbonate, or by continuous addition of alkali such as sodium hydroxide or sodium carbonate, to neutralize free acids formed throughout the digestion. However, the process gives an unsatisfactory pulp yield (51.4 to 61.5 percent, according to the Examples) and the pulp properties are unexceptional. The main improvement is in the waste liquor.

As the Grangaard et a1. patent illustrates, it has not been possible to develop a practical pulping process using oxygen in place of sulfide. A recent investigation of the oxygen digestion process by J. C. Lescot, Essais de delignification de bois feuillus par loxygene en milieu alcalin (Ph.D. Thesis, Univ. of Grenoble, France, Oct. 27, 1967), resulted in the conclusion that alkaline oxygen digestion was not feasible commercially, since the difficulties of impregnation were important, even when using magnesium oxide as a protector.

A serious problem is that a digestion process using oxygen and alkali tends to produce pulp having a dark color, due to a higher lignin content than is present in pulp obtained using a conventional soda cooking process. It is known, for example, that a soda cook using a charge of 20 percent sodium hydroxide based on the wood, a maximum cooking temperature of 165C., and a reaction time of two hours, is capable of producing a cellulose pulp having a lignin content of 4.2 percent. On the other hand, a corresponding oxygen digestion process carried out in the presence of alkali at an oxygen pressure of 8 kg./sq.cm., the alkali charge being 20 percent sodium hydroxide based on the wood, the maximum cooking temperature being 165C, and the reaction time two hours, produces a dark colored cellulose pulp with a lignin content of 18 percent. The oxygen evidently has an undesirable effect on the lignin dissolution, for reasons which cannot be explained.

It has now been discovered that under controlled conditions and moderate oxygen pressures, it is possible to digest wood with a mixture of alkali and oxygen and obtain in high yield a cellulose pulp having a high brightness and a low lignin content. What is essential in order to obtain these results is to limit the amount of alkali at the beginning of the digestion to at most percent, and preferably from about 5 to about 20 percent, of the total molar quantity of alkali required for the digestion, and to add the alkali progressively, either continuously or in increments, during the digestion, while maintaining the pH of the digestion liquor in the course of the digestion within the range from about 9.5 to about 13, and preferably from about 9.5 to about 1 1.5.

Now that it has been discovered that it is possible using alkali and oxygen to produce a cellulose pulp of high brightness and low lignin content, in high yield, it appears possible to explain why these results are obtained under the conditions set forth. This explanation is not offered as a final explanation, since it has not been fully verified by experimental evidence, but it appears to be likely that this is what takes place. Under these conditions, if the amount of alkali is restricted (unlike prior processes in which all of the alkali required for the digestion is added ab initio), condensation of the lignin in the cellulose is avoided, and thus a darkening due to this condensation is prevented, while at the same time the cellulose is protected against excess degradation. Moreover, at the lower alkali concentration, the deleterious effect of oxygen in the production of an oxidative degradation of the cellulose wood components may also be inhibited.

If the pH falls below 9 for an appreciable portion of the digestion process, the brightness of the pulp is deleteriously affected. It is most important that the pH be maintained within the stated range during the first stages and the major portion of the digestion process. With certain types of pulp, particularly viscose pulps and other dissolving pulps, it is possible to permit the pH to drop below 9, down to about 8, during the final stages of the process without seriously affecting the quality of the pulp. Apparently, brightness is affected most by chemical reactions occurring during the initial and major part of the digestion reactions.

The total amount of alkali that is required for the digestion is determined by the quality and type of the pulp to be produced, and is within the range from about 1 to kilomoles per 1,000 kg. of dry wood. It is well known that certain types of pulp are more digested than others. This is entirely conventional, and does not form a part of the instant invention. Cellulose pulps intended to be used in the production of regenerated cellulose fibers, such as viscose, acetate and cuprammonium pulps, are quite fully digested, and should have a low content of lignin and hemicellulose. In the production of such pulps, in accordance with the process of the invention, the amount of alkali can be within the range from about 6 to about 8 kilomoles per 1,000 kg. of dry wood. Semichemical pulps are given an intensive mechanical treatment following their digestion, in order to liberate the cellulose fibers, and in the production of such pulps, using the process of the invention, the amount of alkali can be much less, within the range from about 1 to about 2 kilomoles per 1,000 kg. of dry wood. For the production of bright paper pulp, which is readily defibered when the digester is blown, the amount of alkali used in the process of the invention can be within the range from about 2.5 to about 5 kilomoles. Generally, for most of the types of pulps given an intermediate degree of digestion, such as pulps for fine paper, plastic fillers, and soft paper or tissue paper, the amount of alkali in the process of the invention is within the range from about 2 to about 6 kilomoles per 1,000 kg. of dry wood.

Any alkali metal hydroxide or alkali metal carbonate can be employed, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium carbonate, potassium carbonate and lithium carbonate. The sodium carbonate obtained in the burning of cellulose digestion waste liquors can be used for this purpose. The use of alkali metal carbonates may be more advantageous than the use of alkali metal hydroxides in maintaining the pH of the digestion liquor within the stated range, because of the buffering properties of the carbonate or bicarbonate present or formed in situ. Consequently, mixtures of alkali metal hydroxides and alkali metal carbonates are particularly satisfactory to obtain the advantages of each, and dilute their disadvantages. However, if alkali metal carbonate such as sodium carbonate is the sole alkali charge, the total amount of sodium is greater, and this imposes a greater load on the sodium recovery system.

It is also possible to use mixtures with alkali metal hydroxides or carbonates with alkali metal bicarbonates such as sodiumbicarbonate and potassium bicarbonate. The alkali metal bicarbonate in this case serves as a buffer. Other buffering agents, compounds of alkali metals with nondeleterious acidic anions, can be employed, such as alkali metal acid phosphates, and alkali metal acid or bisulfites, such as potassium dihydrogen phosphate, potassium monohydrogen phosphate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, sodium acid sulfite, and potassium acid sulfite, as well as the lithium salts of these anions.

The amount of buffering agent such as alkali metal bicarbonate is usually within the range from about 1 to about 5 kilomoles per 1,000 kg. of dry wood. The alkali metal bicarbonate or other buffering agent should be added to the digestion liquor either initially or at an early stage of the digestion. The addition of the bicarbonate or other buffering agent increases the buffer capacity of the digestion liquor, thereby assisting in avoiding variations in pH outside the prescribed range during the digestion. The buffering agent, particularly a bicarbonate, is especially desirable when it is desired to operate at a relatively low pH, for example, from about 9.5 to about 9.7. In this case, bicarbonate or other buffering agent can be added to advantage even if alkali metal carbonate is present.

Large amounts of buffering agents, and particularly bicarbonates, should be avoided, however, since the presence of large amounts of additional foreign anions can be undesirable. In the case of bicarbonates, carbon dioxide may be produced in the course of the digestion as the buffer is consumed. The carbon dioxide dilutes the oxygen, and adds an extra load to the chemical recovery system, and is therefore undesirable in large amounts. However, the addition of minor amounts of the buffering agent within the stated range contribute to pulp uniformity because of their assistance in maintaining pH.

It is also important to restrict the amount of bicarbonate or other buffering agents to an amount which is soluble in the digestion liquor, so as to avoid precipitation thereof within the wood being digested.

Also useful as a buffer are the base liquors from previous digestions and/or the waste liquors from oxygen bleaching processes, such as those described in US. applications Ser. Nos. 869,875 and 36,670 referred to above. In this way, better economy is obtained in chemical recovery, which can be effected after evaporating and burning the waste digestion liquor, using known methods.

For economic reasons, the sodium compounds are preferred as the alkali metal hydroxide, alkali metal carbonate and alkali metal bicarbonate.

It is also possible to add the additional chemicals normally present in digestion liquors, such as sodium sulfate, as well as small amounts of sodium sulfide or other alkali metal sulfide. At most, such chemicals are added in an amount of about 1 kilomole per 1,000 kg. of dry wood.

Limiting the amount of alkali metal hydroxide and/or alkali metal carbonate in the initial stages of the pro cess is quite important in obtaining a cellulose pulp of the desired quality. At most, percent of the total molar quantity required of the alkali can be added ab initio, and even this high percentage is only desirable if the pulp to be manufactured is a semichemical pulp, or if the wood has been pretreated with sulfur dioxide in aqueous solution. For most pulps, including even the semichemical pulps, a better cellulose pulp is obtained if the initial charge of alkali is within the range from about 2 to about 50 percent of the total molar quantity required for the digestion. The remainder of the alkali is added progressively, either incrementally or continuously, as the digestion continues. When producing bright pulps having a low lignin content, it is satisfactory to charge not more than 20 percent and suitably from about 5 to about 20 percent of the alkali at the beginning of the digestion process.

If a mixture of alkali metal hydroxide and alkali metal carbonate is used, it is particularly suitable if the initial charge comprises sodium carbonate, optionally with an addition of sodium bicarbonate as described above, the remainder of the alkali added as the digestion proceeds being sodium hydroxide. If the alkali charge initially is alkali metal hydroxide, it is usually important in producing pulps having a low lignin content that the initial charge be low, within the range from about 2 to about percent, of the total molar quantity of alkali.

Whether or not the digestion process is carried out continuously or as a batch process, the alkali metal hydroxide and/or alkali metal carbonate can be charged continuously or in increments to the digestion liquor. in a continuous digestion, the wood is caused to move through the digester from one end to the other which thereby constitutes a reaction zone. In a batch process, the wood, usually in the form of chips, is retained in the reaction vessel throughout the digestion.

Since the oxygen that is employed as an essential component in the digestion process of the invention is a gas, the so-called gas phase digestion procedure can be used to advantage. In this case, the wood and the film of digestion liquor present on the wood are kept in continuous contact with the oxygen-containing gas. If the wood is completely or substantially immersed in the digestion liquor, it is important to agitate the wood and- /or the gas and/or atomize the gas or the liquor. The oxygen should be dissolved or dispersed in the digestion liquor to the greatest extent possible. Dissolution or dispersion of the oxygen in the liquor can take place within the digestion vessel and/or externally of the same, such as in nozzles, containers or other known devices used for dissolving or dispersing gases in liquids.

Transfer of oxygen to the wood material impregnated with digestion liquor is important in the process, and is controlled by adjusting the oxygen pressure, the digestion temperature and/or the proportion of gas-liquid contact surfaces, including the wood impregnated with digestion liquor.

The oxygen is preferably employed as pure oxygen, but mixtures of oxygen with other inert gases can be used, such as, for example, mixtures of oxygen with nitrogen and with carbon dioxide and with both, as well as air. Compressed air can also be used, although this complicates the devices for dissolving or dispersing the oxygen in the reaction mixture.

Prior to contact with the oxygen, the wood suitably in the form of chips can be impregnated with an aqueous digestion liquor containing the desired chemicals. The chips are impregnated under vacuum, or under atmospheric pressure or superatmospheric pressure, or by other methods conventional in wood digestion processes. The wood may also be treated with steam before being brought to the digestion zone.

The temperature employed during the impregnation can be within the range from about 20 to about 120C., although temperatures within the range from 90 to 120C. would not normally be used except under special circumstances. In the latter case, the highest temperature during the digestion may be the same as the impregnating temperature, as well as the initial digestion temperature. Generally, however, it is to advantage if the digestion temperature is allowed to rise during the digestion process, so that normally the temperature during impregnation would not exceed about 60C.

The digestion can be carried out at a temperature within the range from about 60 to about 175C. Usually, it is advantageous if the digestion temperature is permitted to rise during the digestion process from an initial temperature of the order of from 60 to C. to the maximum digestion temperature, of the order of from to or C.

At a maximum temperature of 90C., the digestion process proceeds slowly, but on the other hand, moderate oxygen pressure and simple technical apparatus can be used. A digestion temperature of from 90 to 110C. can be used to advantage when producing semichemical pulps, the fibers of which are not fully liberated until after subjection to a mechanical treatment process, such as in a refiner after the digestion process. These are high yield pulps.

If a maximum digestion temperature of from 150 to 175C. is used, the digestion will proceed rapidly. On the other hand, in this case, an exceedingly effective transfer of oxygen to the wood from the gas phase is required. This requires intimate contact and high oxygen pressure. By effective control methods, however, all of which are conventional, it is possible to control the digestion within the temperature range, particularly. when producing cellulose pulp of moderate yield.

Normally, a maximum digestion temperature within the range from 110 to 150C. is preferred, at which temperature the digestion can take place in a reasonable time using relatively simple apparatus and under moderate oxygen pressure, with good control of pulp quality, irrespective of whether semichemical pulps are being produced or cellulose pulps whose fibers can be liberated without intensive mechanical treatment, or are simplyliberated when the cooker or digestion vessel is blown.

The partial pressure of oxygen during the digestion process should be within the range from about 1 to about 20 atmospheres, preferably from about 3 to about 20 atmospheres. Higher pressures should not be used, from the standpoint of safety, and are definitely unnecessary. At lower pressures, the digestion proceeds more slowly, and such pressures are not economically practical. Normally, a pressure within the range from about 3 to about 12 atmospheres is preferred.

Because of the consumption of oxygen in the course of the digestion, and the higher rate at which the digestion proceeds at high reaction temperatures, it follows that the higher the reaction temperature, the higher the pressure that should be applied during the reaction. The optimum temperature and pressure conditions for a given pulp can be determined by digestion sampling procedures, as is well known. Such trial-and-error experimentation is conventional, and is not a part of this invention.

Pulps for a certain field of use, for example, for use in the production of paper, should have a high degree of strength. In such cases, it is suitable to carry out the digestion in the presence of an inhibitor or mixture of inhibitors which protect the cellulose and hemicellulose molecules against uncontrolled degradation. The effect of the inhibitors is reflected by the viscosity of the pulp, and the degree of polymerization of the cellulose.

The inhibitors can to advantage be charged to the digestion liquor during an early stage of the digestion or, preferably, at the beginning, before the digestion heating is begun. Thus, they can be added to the digestion liquor before combination with the wood, or shortly thereafter. Suitable inhibitors are water-insoluble magnesium compounds, such as magnesium carbonate. Magnesium carbonate is known, and is disclosed in U.S. Pat. No. 3,384,533 to Robert et a1. dated May 21, 1968 as useful in the delignification and bleaching of cellulose pulps with alkali and oxygen, but this is not a digestion of wood. Other water-insoluble magnesium compounds such as magnesium oxide and hydroxide are disclosed in South African Pat. No. 3771/68 to LAir Liquide, also relating to alkaline oxygen bleaching of cellulose pulps. Also useful are water-soluble magnesium compounds such as magnesium chloride or magnesium acetate, which form water-insoluble magnesium compounds in the alkaline digestion liquor such as magnesium hydroxide or magnesium carbonate, and therefore exist as such insoluble compounds in the course of the digestion. These are also disclosed in South African Pat. No. 377 l//68. However, magnesium compounds which are soluble in the digestion liquor in the course of the digestion process are preferred. Such are the water-soluble complex magnesium compounds.

Aliphatic alpha-hydroxycarboxylic acids of the type RCHOHCOOH and the corresponding beta-hydroxycarboxylic acids RCHOHCH COOH have the property of forming chelates with magnesium. These chelates are of the type:

In the formula, n is zero or one. When n is zero, the acid is an alpha-hydroxy acid, and when n is one, the acid is a beta-hydroxy acid.

R in the above formula is hydrogen or an aliphatic radical, which may be a hydrocarbon radical having from one to about ten carbon atoms, or a hydroxysubstituted hydrocarbon radical having from one to nine hydroxyl groups, and from one to about ten carbon atoms.

Exemplary alpha-and beta-hydroxy carboxylic acids are glycolic acid, lactic acid, glyceric acid, :,B-dihydroxybutyric acid, a-hydroxy-butyric acid, a-hydroxyisobutyric acid, a-hydroxy-n-valeric acid, a-hydroxyisovaleric acid, B-hydroxy-butyric acid, B-hydroxyisobutyric acid, B-hydroxy-n-valeric acid, B-hydroxyisovaleric acid, erythronic acid, threonic acid, trihydroxy-isobutyric acid, and sugar acids and aldonic acids, such as gluconic acid, galactonic acid, talonic acid, mannonic acid, arabonic acid, ribonic acid, xylonic acid, lyxonic acid, gulonic acid, idonic acid, altronic acid, allonic acid, ethenyl glycolic acid, and ,B-hydroxy-isocrotonic acid.

Also useful are organic acids having two or more carboxylic groups, and no or from one to ten hydroxyl groups, such as oxalic acid, malonic acid, tartaric acid, malic acid, and citric acid, ethyl malonic acid, succini acid, isosuccinic acid, glutaric acid, adipic acid, suberic acid, azelaic acid, maleic acid, fumaric acid, glutaconic acid, citramalic acid, trihydroxy glutaric acid, tetrahydroxy adipic acid, dihydroxy maleic acid, mucic acid,

mannosaccharic acid, idosaccharic acid, talomucic acid, tricarballylic acid, aconitic acid, and dihydroxy tartaric acid.

Magnesium complexes of nitrogen-containing polycarboxylic acids are especially effective inhibitors. Several important acids belonging to this group have the formula:

l-lOOCCH A or alkali metal salts thereof, in which A is the group CH COOH or CH CH OH, where n is an integer from zero to five. The mono, di, tri, tetra, penta and higher alkali metal salts are useful, according to the available carboxylic acid groups converted to alkali metal salt form.

Examples of such compounds are ethylenediaminetetraacetic acid, ethylene diamine triacetic acid, nitrilotriacetic acid, diethylene-triaminopentaacetic acid, tetraethylenepentamine heptaacetic acid, and hydroxyethylethylenediaminetriacetic acid, and their alkali metal salts, including the mono, di, tri, tetra and penta sodium, potassium and lithium salts thereof. Other types of aminocarboxylic acids which can be used to advantage are iminodiacetic acid, 2-hydroxyethyliminodiacetic acid, cyclohexanediaminetetraacetic acid, anthranil-N,N-diacetic acid, and 2-picolylamine- N,N-diacetic acid.

These complexing agents can be present in rather large quantities, within the range from about two to about 10 times the amount needed to prevent precipitation of magnesium hydroxide during the digestion. The use of waste digestion liquor in combination with complexing agents of this type is particularly advantageous.

The polyphosphoric acids are also good complexing agents for magnesium, and the magnesium salts of these acids are useful in the process of the invention. Exemplary are disodium-magnesium pyrophosphate, trisodium-magnesium tripolyphosphate and magnesium polymetaphosphate.

Especially advantageous from the standpoint of cost are the acids naturally present in waste liquors obtained from the alkaline treatment of cellulosic materials. These acids represent the alkali-or water-soluble degradation products of polysaccharides which are dissolved in such liquors, as well as alkali-or water-soluble degradation products of cellulose and hemicellulose. The chemical nature of these degradation products are complex, and they have not been fully identified. However, it is known that saccharinic and lactic acids are present in such liquors, and that other hydroxy acids are also present. The presence of C -isosaccharinic and C -metasaccharinic acids has been demonstrated, as well as C.,- and C metasaccharinic acids. Glycolic acid and lactic acid are also probable degradation products derived from the hemicelluloses, together with betagamma-dihydroxy butyric acid.

Carbohydrate acid-containing cellulose waste liquors which can be used include the liquors obtained from the hot alkali treatment of cellulose; liquors from sulfite digestion processes; and liquors from sulfate digestion processes, i.e., kraft waste liquor. The waste liquors obtained in alkaline oxygen gas bleaching processes, for example, those disclosed in Ser. Nos. 869,875 and 36,670, or alkaline peroxide bleaching processes can also be used. In this instance, the alkaline liquor can be taken out from the process subsequent to completing the oxygen gas treatment stage, or during the actual treatment process.

The complex magnesium salts can be formed first, and then added to the digestion liquor. They can also be formed in situ from a water-soluble or waterinsoluble magnesium salt, oxide or hydroxide, in admixture with the complexing acid, and this mixture can be added to the digestion liquor. Preferably, the waste liquor employed as the source of complexing acid or lactone or salt thereof can be mixed with a magnesium salt, oxide or hydroxide, before being introduced to the process. It is also possible to add the magnesium salt, oxide or hydroxide to the digestion liquor, and then being the liquor into contact with the complexing acid or lactone or salt thereof. It is also possible to combine the complexing acid or lactone or salt thereof with the liquor and then add the magnesium salt, oxide or hydroxide, but this method may be less advantageous in practice.

In whatever form the magnesium is added, whether as salt, oxide, hydroxide, or complex salt, the amount of magnesium is calculated as MgO.

A noticeable improvement is obtained when as little magnesium as 0.01 percent MgO, calculated on the dry weight of the wood, is added. A high proportion of magnesium, up to 1 percent MgO, calculated on the dry weight of the wood, has been employed without disadvantageous effect. However, for economic reasons, it is usually desirable to use as little magnesium as possible, and preferably an amount within the range from about 0.05 percent to 0.5 percent MgO, calculated on the dry weight of the wood, is employed.

Upon conclusion of the alkaline oxygen gas digestion, it is possible to separate the magnesiumcontaining waste liquor and recycle if for reuse. The consumption of magnesium salts is negligible, and usually it is not even necessary to replenish the magnesium content before recycling. However, additional magnesium compound can be added before recycling, if necessary, to restore the magnesium content, as MgO, and maintain a high enough-level, for instance, to prevent oxidative degradation of the cellulose or hemicellulose. The consumption of magnesium salt has been noted to be particularly low when waste liquor from a part of the alkaline oxygen gas treatment process is employed as the source of complexing acid, and recycled for continued treatment of new batches of wood.

Some waste liquors are particularly high in magnesium ion because of the nature of the pulp or of the pulping process. For example, liquors from the cooking of wood with magnesium bisulfite or magnesium sulfite usually contain enough magnesium ion so that no addition of magnesium compound need be made. Such waste liquors can be used per se, in the process of the invention, inasmuch as they already contain the complexing acids, and a sufficient proportion of magnesium ion as well.

As a source of magnesium, one may add any magnesium salts, oxide or hydroxide, either to regenerate a spent treatment liquor, or to prepare a waste liquor or other material for use in the process. Any water-soluble magnesium compound can be used, such as for example, magnesium sulfate, magnesium chloride, magnesium bromide, magnesium chlorate, magnesium potassium chlorate, magnesium formate, magnesium oxide, magnesium acetate, magnesium hydroxide, and magnesium nitrate. If it is desired to recover the liquor after the treatment, then it is usually preferable to employ magnesium sulfate, so as to avoid the introduction of corrosive anions into the system. Magnesium compounds which have no deleterious anion or which have an anion which is destroyed in the course of the process, such as magnesium oxide, magnesium hydroxide, and magnesium carbonate, are also advantageous. Since these are water-insoluble, it is desirable, however, to combine these with the complexing agent in the presence of water, and await their dissolution, indicating that the complex has been formed, before combining with the digestion liquor, or before commencing the alkaline oxygen gas digestion. Any other waterinsoluble magnesium compounds can be used in this way, for instance, magnesium phosphate, magnesium silicate and magnesium sulfide.

Also effective inhibitors are the alkali-soluble silicic acids or silicates, preferably the alkali metal silicates, such as sodium silicate and potassium silicate. The amount usually is within the range from about 0.01 to about 2 percent, calculated as SiO by weight of the dry wood, but preferably the amount is within the range from about 0.05 to about 1 percent by weight of the dry wood.

The protective effect is particularly great if both magnesium compounds and silicic acid are used as the inhibitors.

It is often suitable during the digestion to withdraw a portion of the digestion liquor, such as by draining, pressing, displacement or filtering. This liquor can be returned to the digestion process at a later stage, or to a subsequent batch, and in this event it is advantageous to heat the liquid or a part thereof under pressure to an elevated temperature of the order of from 1 10 to about 200C. in intimate contact with an oxygencontaining gas such as air in order to oxidize organic substances in the liquor. The liquor can be fortified by adding alkali metal hydroxide and/or alkali metal carbonate and/or inhibitor before or after pressureheating.

In the manufacture of many types of cellulose pulp, particularly viscose pulp, cuprammonium pulp, and paper pulp having a high degree of opacity, it has been found advantageous to pretreat the wood before the digestion with water or an aqueous acidic, neutral, or alkaline solution, optionally in several stages. In the case of such pulps, this pretreatment is preferably effected at a high temperature, within the range from about to about l50C., whereupon a marked dissolution of from about 2 to about 15 percent of the wood material takes place. Waste liquor from wood cellulose alkaline treatment processes can be used as the alkaline medium, and the treatment may be continued until the pH of the solution drops and the solution becomes acidic. It has been found particularly suitable to use in this pretreatment the alkaline waste liquor removed subsequent to or during the oxygen digestion process of the instant invention.

When producing high strength paper pulps, the pretreatment can be carried out with such solutions at lower temperatures, from about 30 to about C, preferably using an acid solution such as a 0.1 to l percent aqueous solution of sulfuric acid, nitric acid, or

ill

phosphoric acid. These acids can also be used in high temperature pretreatment.

Before carrying out the oxygen digestion process of the present invention, it is particularly suitable to pretreat the wood with an aqueous solution containing sulfur dioxide, sodium bisulfite and/or sodium sulfite or other alkali metal sulfite such as potassium bisulfite or sulfite. The treatment causes some dissolution and modification of the wood material, which has been found to be favorable during the oxygen digestion, particularly in the case of wood material which is difficult to pulp without any form of pretreatment, such as softwood. By pretreating the wood in this manner with water or aqueous solutions, the pulp can be modified to any desired degree, and by suitably selecting the conditions according to trial-and-error experimentation (which can be carried out on a small sample) to suit the wood used, the treating conditions can be optimized for different fields of use of the pulp product. Generally speaking, the pretreatment causes a reduction in the consumption of alkali during the oxygen digestion process of the invention.

When producing many types of pulps which should be metal-free, or substantially so, such as viscose and cuprammonium pulps and pulps for high strength papers, the pretreatment stage or part thereof is preferably carried out in the presence of a complexing agent for bivalent and/or polyvalent metal ions, such as copper, iron, manganese, cobalt and vanadium. In this way, it is possible to remove and/or render harmless ions of the so-called transition metals, which catalyze an oxidative degradation of the carbohydrates during the subsequent digestion process. Examples of suitable complexing agents are chelating salts of nitrogencontaining polycarboxylic acids of the class set forth above in conjunction with the magnesium complex as well as polyphosphates and ethylenediamine and ethylenediamine derivatives, although other complexing agents of an inorganic or organic nature can also be used to advantage. The effect can be increased if mixtures of different complexing agents are used, since certain complexing agents having more of an affinity for certain polyvalent metal ions than others, and a blend is better capable of chelating a mixture of polyvalent metal ions for this reason. The use of complexing agents in connection with the pretreatment has been found to promote uniformity of the pulp during digestion.

It may be desirable to wash the wood with water between the pretreatment stage and the oxygen digestion process. This washing step may be desirable in the case of any of the pretreatment processes described above. The washing, however, increases the cost of the processing, and also increases the risk of water contamination of the pulp with metal ions and metal compounds, and consequently it may often be more practical to omit the washing step, unless it can be carried out with deionized water, at low cost. Omission of the washing is usually disadvantageous.

[t has also been found advantageous to have complexing agents for bivalent and/or polyvalent metal ions present during the oxygen digestion process. Any complexing agents which are stable and not deleteriously affected by the digestion liquor can be used. Suitable complexing agents include those mentioned above.

A surface-active agent can be added to the digestion liquor, and contributes to a reduction in the resin content of the wood cellulose produced from the wood. This also surprisingly contributes to a reduction in the lignin content, and a more uniform delignification. The surface-active agent is suitably added at the beginning of the digestion process, or during an early stage of the digestion, and may be present during all or only a part of the digestion. Cationic, anionic, and nonionic surface-active agents and mixtures thereof can be used. If liquor is circulated during the digestion process, it is suitable to use agents which do not produce foam. Examples of suitable surface-active agents are polyalkylene glycol ethers of fatty alcohols and alkyl phenol polyoxyalkylene glycol ethers. Sulfonated anionic surface-active agents such as the alkylbenzene sulfonates can also be used.

Also suitable are nonsurface-active quaternary ammonium lower alkyl and/or lower alkanol and/or polyoxyalkylene alkanol salts which have the formula:

In the above formula, from one to four of R R R and R are saturated aliphatic hydrocarbon radicals having from one to about four carbon atoms; and/or from one to four of R R R and R are hydroxyalkyl or polyoxyalkylene radicals terminating in a hydroxyl group, and having a formula selected from the group consisting of (C H O),,,H, (C H O),,l-l and (C H O),,H, wherein m is an integer from zero to five, p is an integer from zero to five, and q is an integer from zero to two; and mixtures of two or more thereof. Thus, all of the R radicals are either saturated lower aliphatic hydrocarbon radicals or hydroxyalkyl or hydroxyalkylene polyoxyalkylene radicals of these types.

X is an inorganic anion, and is preferably selected from the group consisting of H CH SO C H SO Cl and Br. The nature of X is not critical, provided it is inert in the cellulose pulping liquor.

It will be evident that these compounds are quaternary lower hydrocarbon amines having methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl and tertbutyl groups, in any combination of the same and different groups; quaternary alkanol amines, and quaternary hydroxyalkylene amines having polyoxyethylene, polyoxypropylene, and/or polyoxybutylene groups; or quaternary hydrocarbon alkanol amines having mixed combinations of such groups.

Exemplary quaternary ammonium compounds are tetramethyl ammonium chloride, trimethylethyl ammonium bromide, monopropyl dimethyl ethyl ammonium chloride and ammonium dibuty] methyl monomethyl sulfate.

The preferred compounds are quaternary methyl triethanolamines having the formula:

The quaternary hydrocarbon amines are known, and are available commercially. Where not available, they are readily obtainable by known methods.

The quaternary triethanolamines, tripropanolamines and tributanol amines also are known, and they and their polyoxyalkylene derivatives can be prepared in known manner by the condensation of ethylene oxide, propylene oxide or butylene oxide with ammonia or a mono, di or trialkanolamine. The reaction mixture thus can contain mixtures of mono-, diand trialkanolamines, together with higher polyoxyalkylene derivatives. This mixture can be subjected to quaternization, but it is preferable to distill the product, so as to remove the trialkanolamine fraction in the form of a product having from 98 to 99 percent trialkanolamine.

The quaternary ammonium nonsurface active resin control agent can be prepared from the alkanolamine such as 98-99 percent triethanolamine by quaternizing with the quaternizing agent, such as dimethyl sulfate, in known manner.

The oxygen digestion process of the invention is applicable to any kind of wood. in general, hardwood such as beech and oak can be pulped more easily than softwood, such as spruce and pine, but both types of wood can be pulped satisfactorily using this process. Exemplary hardwoods which can be pulped include birch, beech, poplar, cherry, sycamore, hickory, ash, oak, chestnut, aspen, maple, alder and eucalyptus. Exemplary softwoods include spruce, fir, pine, cedar, juniper and hemlock.

1n the case of softwood, the processing conditions, including the particle size of the wood fragments, the digestion temperature, the alkali concentration, and the oxygen pressure, should be carefully determined and controlled during the digestion.

The wood should be in particulate form. Wood chips having dimensions that are conventionally employed in the sulfate process can be used. However, appreciable advantages with respect to uniformity of the digestion process under all kinds of reaction conditions within the stated ranges can be obtained if the wood is in the form of nonuniform fragments of the type of wood shavings or chips having an average thickness of at most 3 mm., and preferably within the range from about 0.2 to about 2 mm. Other dimensions are not critical. Sawdust, wood flour, wood slivers and splinters, wood granules, and wood chunks, and other types of wood fragments can also be used. It is important, particularly in the case of softwood, that the wood fragments be thin, since otherwise the digestion may be nonuniform, and the process may be more difficult to control.

After the oxygen digestion process has been completed, the pulped wood may optionally be subjected to a mechanical treatment in order to liberate the fibers. If the pulping is brief or moderate, a defibrator, disintegrator, or shredder may be appropriate. After an extensive or more complete pulping or digestion, the wood can be defibrated in the same manner as in other conventional cellulose cooking processes, such as sulfate pulping, by blowing off the material from the digester, or by pumping.

The pulped wood cellulose that is obtained in accordance with the process of the invention is of such whiteness that it can be used to advantage directly for producing tissue paper, light cardboard and magazine paper. When a higher degree of brightness is desired,

as for fine paper, rayon and cellulose derivatives, the pulp can easily be bleached in accordance with known methods by treatment with chlorine, chlorine dioxide, chlorite, hypochlorite, peroxide, peracetate, oxygen or any combinations of these bleaching agents in one or more bleaching sequence as described in for example US. application Ser. No. 882,812, now US. Pat. No. 3,652,388. Chlorine dioxide has been found to be a particularly suitable bleaching agent for the oxygen digested cellulose pulp obtained in accordance with this invention. The consumption of bleaching chemicals is generally markedly lower in bleaching oxygen digested pulps of the invention than when bleaching sulfate cellulose.

The chemicals used for the digestion process can be recovered after the waste liquor is burned and subsequent to optionally causticizing all or part of the carbonate obtained when burning the liquor.

Preferred embodiments of the digestion process of the invention and of the cellulose pulps of the invention are shown in the following Examples:

EXAMPLE 1 Birch chips about 1 mm. thick and having a lignin content of 21.1 percent were used in this example. a. As a reference control, the chips were treated in an alkaline aqueous solution under the following conditions:

Oxygen partial pressure, atmospheres 8 initial temperature, C Initial pH 13.4 Time for temperature rise from 80 to C, minutes 45 Wood-to-liquid ratio, kg/l 1 :4 Alkali charge, NaOH kmol/1000 kg dry wood, all added at the start, before heating was begun 5 At 120C waste liquor was drawn off so that a wood-toliquid ratio of 1:2 was obtained pH after 45 minutes 1 1.5 Time digested at 120C after liquid withdrawal, minutes 360 Total digestion time 405 pH after 405 minutes 10.8 The digestion was conducted in an autoclave rotating in a heated glycol bath. After the digestion, the burnt chips residue was washed. A 63.4 percent yield was obtained. The burnt chips residue was dark brown in colour, and the lignin content after digestion was 20.4 percent. b. In a digestion carried out in accordance with the invention, the alkali addition was conducted in four stages. 1.25 kmol of NaOH based 1,000 kg of dry wood was charged in the dirst stage, and in each of the three following stages 0.75 kmol of NaOH was charged. The digestion was otherwise carried out as before, with the exception that the alkali charge was reduced in accordance with the aforegoing. The digestion time at each stage was 240 minutes. The cellulose material was washed with water between each treatment stage. Subsequent to impregnating the wood with cooking liquor, the pH was 13, but fell to 10 in the liquor which was withdrawn the first time at 120 C. The pH of the liquors withdrawn in the latter stages was within the range from 10 to 11. i The lignin content of the pulp after the digestion was 1.5 percent, the pulp yield was 51.5 percent, and pulp brightness 75 percent SCAN.

c. In another digestion, the wood was digested as in b), but not under oxygen pressure. After the digestion, the burnt chips residue had a lignin content of 22.1 percent, and the yield was 71 percent. The chips residue was dark brown in colour.

The results in a), b) and c) show that if the desired delignification effect is to be achieved at a pH of approximately 9 to approximately 13, it is necessary that:

a. the digestion be effected under oxygen pressure b)v.c).)

b. the alkali charge be made in several stages, and not all ab initio b)v.a).)

Experiments a) and c) resulted in burnt chips. The pulp obtained in b) had a particularly high brightness.

EXAMPLE 2 Beech chips were treated in accordance with Example lb) but with the addition of a magnesium complex prepared from MgSO and ethylene-diaminetetraacetic acid (EDTA). The magnesium complex was added prior to the heating. The quantity of magnesium (as MgO) corresponded to 0.1 percent based on the dry wood. The amount of EDTA was 0.5 percent based on the dry wood. In this instance, the pH values during the oxygen digestion were approximately 0.5 unit higher than in Example lb). This is related to the fact the yield of carbohydrates was higher, due to the presence of the magnesium complex. The lignin content of the pulp was 2 percent, pulp yield was 57.5 percent, and the brightness of the pulp was the same as that in Example lb). The results show that the yield is greatly improved by the addition of magnesium complex.

EXAMPLE 3 a. Birch chips having a lignin content of 2 0.9% and a thickness of 1.5 mm were digested using sodium carbonate as the alkali under the following conditions: Pretreatment: waste digestion liquor from Example lb).

lmpregnating temperature, C 80 Digestion:

Oxygen partial pressure, atmospheres 8 Initial temperature, C 80 Initial pH 11 Time for temperature to rise from 80 to 120C, minutes 45 Wood-to-liquid ratio, kg/l 1:4 pH after 45 minutes 9.9 First Na CO charge, 2.5 kmoles per 1,000 kg dry wood at 120C waste liquor was drawn off so that a wood-toliquid ratio of 1:2 was obtained Time at 120C after withdrawing liquid, minutes 90 pH after 135 minutes 9.5 Additional Na CO solution was then injected so that the wood-to-liquid ratio increased to 1:4 Second Na CO charge, 1.25 kmoles to 1,000 kg dry wood After 45 minutes, liquid was drawn off at 120C so that a wood-to-liquid ratio of 1:2 was obtained Time at 120C after withdrawing liquid, minutes 180 pH after 360 minutes 9.7 Injection of Na CO solutiuon was repeated a further two times in corresponding manner as the previous injections, with 0.75 kmole Na CO per 1,000 kg wood at each charge, and 45 minutes heating at 120C thereafter. pH after 540 minutes 9.9

Prior to being digested, the wood was impregnated with cooking liquor. The digestion was effected in a rotary autoclave. After each digestion stage, the pulp was washed with water. Subsequent to impregnating the wood with cooking liquor at C, the pH was 1 1.0. During the major portion of the oxygen cooking process, the pH remained within the range 9.5 to 10. The lignin content of the pulp was 3 percent, the pulp yield 56.4 percent, and the pulp brightness 64 percent SCAN. The results show that a good delignification is obtained if sodium carbonate is used. b. The process was repeated exactly as in a), but without washing the wood between the different digestion stages. The same results were obtained.

EXAMPLE 4 Pine chips having a lignin content of 28 percent and 1.5 mm thick were pretreated with an aqueous bisulphite solution containing 20 grams of sodium bisulphite per liter, under the following conditions:

Pretreatment:

Treatment temperature, C

Tre t ent mstminu e Wood-to-liquid ratio, kg/I 4 1:4 I if Digestion:

Oxygen partial pressure, atmospheres 8 Initial temperature, C 80 Initial pH 11 Time for temperature to rise from 80C to 120C,

minutes 35 Wood-to-liquid ratio, kg/ 1 1 :4

pH after 45 minutes 10.5 First NaOH charge, 1.25 kmoles per 1,000 kg dry wood At 120C waste liquor was drawn off so that a wood-toliquid ratio of 1:2 was obtained Time at 120C after withdrawing liquid, minutes pH after minutes 9.3 Additional NaOH solution was then injected so that the wood-to-liquid ratio increased to 1:4 Second NaOH charge 1.25 kmoles to 1,000 kg dry wood After 45 minutes, liquid was drawn off at 120C so that a wood-to-liquid ratio of 1:2 was obtained Time at 120C after withdrawing liquid, minutes pH after 360 minutes 9.5 Injection of NaOH solution was repeated a further three times in corresponding manner as the previous injections, with 1.25 kmole NaOH per 1,000 kg wood at each charge, and 45 minutes heating at 120C thereafter.

pH after 630 minutes 9.7 Prior to being digested, the wood was impregnated with cooking liquor. The digestion was effected in a rotary autoclave. After each digestion stage, the pulp was washed with water.

Subsequent to impregnating the wood with cooking liquor at 80C, the pH was 10.5. During the major portion of the oxygen cooking process, the pH remained within the range 9.2 to 10.5. After the digestion, the

pulp was washed with water. The lignin content of the pulp was 3.1 percent, the pulp yield was 48.6 percent, and the pulp brightness 55 percent SCAN. The results show that it is possible to produce pulps of high brightness and low lignin content in accordance with the invention, when pulping softwood.

EXAMPLE Spruce chips having a lignin content of 27.6 percent and a thickness of 1.6 mm were pretreated with an aqueous bisulphite solution containing 20 grams of sodium bisulphite per liter and disodium salt of ethylendiaminetetraacetic acid in a quantity corresponding to 0.4 percent based on the dry weight of the wood:

Pretreatment temperature, C 120 Pretreatment time, minutes 90 Wood-to-liquid ratio, kg/ 1 1:4

pH 5 After the pretreatment, the chips were washed with water, and then treated in five stages according to the following:

Oxygen partial pressure, atmospheres 8 lmpregnating temperature, C 80 Time for temperature to rise to 120C 45 Wood-to-liquid ratio kg/ 1 1:4

charge, kmoles per 1,000 kg dry wood 1.25

Bicarbonate charge, on wood 2 pH after 45 minutes 10 At 120C liquor was withdrawn so that a wood-toliquid ratio of 1:2 was obtained Time atlC after withdrawing liquor, minutes pH after 105 minutes 9.5 Further NaOH solution (1.25 kmoles) was then injected so that the wood-to-liquid ratio increased to 1:4

After 45 minutes, liquor was withdrawn so that the wood-to-liquid ratio of 1:2 was obtained pH after 150 minutes 10.2

Temperature, C 120 Time at 120C after tapping said liquor, minutes 180 Injection of NaOH solution was repeated a further three times in the same manner as with previous injections, with 1.25 kmole NaOH per 1,000 kg wood at each charge, and 45 minutes heating at 120C thereafter.

pH after 455 minutes -9.8 The digestion was carried out in a rotary autoclave. Subsequent to digestion, the pulp was washed with water. The pH of the cooking liquor was within the range of 9.5 to 10 during the major portion of the process. The lignin content of the pulp was 1.6 percent, the pulp yield was 46.5 percent, and the pulp brightness 62 percent SCAN. The results show that using softwood it is possible to obtain a good degree of delighification and a high degree of brightness in good pulp yield by using complexing agents for heavy metal ions during the pretreatment and bicarbonate during the oxygen digestion.

We claim:

1. 1n the process for the digestion of wood in a digestion liquor comprising a mixture of aqueous alkali and oxygen, the improvement which results in the obtention is high yield of a cellulose pulp having a high brightness and a low lignin content, which comprises limiting the amount of alkali at the beginning of the digestion to at most 75 percent of the total molar quantity of alkali required for the digestion, and adding the remainder of the required alkali progressively, during the digestion, while maintaining the pH of the digestion liquor in the course of the digestion within the range from about 9.5 to about 13 during the first stages and the major portion of the digestion process.

2. The process of claim 1, in which the amount of alkali is within the range from about 5 to about 20 percent of the total molar quantity of alkali required.

3. The process of claim 1, in which the alkali is added continuously during the digestion.

4. The process of claim 1, in which the alkali is added in increments during the digestion.

5. The process of claim 1, in which the pH is within the range from about 9.5 to about 11.5.

6. The process of claim 1, in which pH is permitted to drop below 9, down to about 8, during the final stages of the process without seriously affecting the quality of the pulp.

7. The process of claim 1 in which the total amount of alkali is within the range from about 1 to about 10 kilomoles per 1,000 kg. of dry wood.

8. The process of claim 7, in which the cellulose pulp produced is a viscose, acetate or cuprammonium pulp, and the amount of alkali is within the range from about 6 to about 8 kilomoles per 1,000 kg. of dry wood.

9. The process of claim 7 in which the cellulose pulp produced is a semichemical pulp, and the amount of alkali is within the range from about 1 to about 2 kilomoles per 1,000 kg. of dry wood.

10. The process of claim 7, in which the cellulose pulp produced is a paper pulp, and the amount of alkali is within the range from about 2.5 to about 5 kilomoles per 1,000 kg. of dry wood.

11. The process of claim 7, in which the cellulose pulp produced is a pulp for use in making fine paper, plastic fillers, and soft paper or tissue paper, and the amount of alkali is within the range from about 2 to about 6 kilomoles per 1,000 kg. of dry wood.

12. The process of claim 1 in which the alkali is any alkali metal hydroxide or alkali metal carbonate.

13. The process of claim 12 in which the alkali metal hydroxide is sodium hydroxide.

14. The process of claim 12 in which the alkali metal carbonate is sodium carbonate.

15. The process of claim 1 in which the alkali is a mixture of alkali metal hydroxide or alkali metal carbonate with an alkali metal bicarbonate which serves as a buffer.

16. The process of claim 15 in which the alkali metal bicarbonate is sodium bicarbonate.

17. The process of claim 16 in which the amount of alkali metal bicarbonate is within the range from about 1 to about 5 kilomoles per 1,000 kg of dry wood.

18. The process of claim 1 in which the aqueous alkali includes a buffering agent which increases the buffer capacity of the digestion liquor, and assists in maintaining the pH within the prescribed range during the digestion.

19. The process of claim 18, in which the pH is maintained within the range from about 9.5 to about 9.7.

20. The process of claim 18, in which the buffering agent is a waste liquor selected from the group consisting of waste liquor from a previous alkaline oxygen digestion and waste liquor from an alkaline oxygen bleaching process.

21. The process of claim 1, in which the initial alkali charged comprises an alkali metal carbonate and the remainder of the alkali charged as the digestion proceeds is an alkali metal hydroxide.

22. The process of claim 21 in which the initial alkali charged also includes an alkali metal bicarbonate.

23. The process of claim 1, in which the alkali comprises a mixture of an alkali metal hydroxide and alkali metal carbonate.

24. The process of claim 1, in which the digestion temperature is within the range from about 90 to about 175C.

25. The process of claim 1, in which, during the major portion of the digestion, the partial pressure of oxygen is within the range from about 3 to about 12 atmospheres.

26. The process of claim 1, in which the aqueous alkali comprises an inhibitor inhibiting attack on the hemicellulose and cellulose.

27. The process of claim 26, in which the inhibitor is a magnesium compound.

28. The process of claim 27, in which the magnesium compound is a magnesium complex selected from the group consisting of an inorganic or organic acid complex of polyhydroxy acids, organic acids containing at least two carboxylic acid groups, and polyphosphoric acids.

29. The process of claim 28, in which the organic acid is an aliphatic a or ,8 -hydroxy carboxylic acid.

30. The process of claim 26, in which the inhibitor is an alkali-soluble silicic acid or alkali metal silicate.

31. The process of claim 1, in which the wood is pretreated with water or an aqueous acidic, neutral or alkaline solution before the digestion.

32. The process of claim 31, in which the aqueous solution comprises a sulfite precursor selected from sulphur dioxide, an alkali metal bisulphite, or an alkali metal sulphite.

33. The process of claim 31, in which the pretreatment is carried out in the presence of a complexing agent for bivalent and or polyvalent ions of transition metals.

34. The process of claim 1, in which a surface-active substance is present during at least a part of the digestion process.

35. The process of claim 1, in which the wood is a hardwood selected from birch, beech, poplar, aspen, maple, alder and eucalyptus.

36. The process of claim 1, in which the wood is in the form of particles having an average thickness of at most 3 mm.

UNITED Sl'ATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,769,152 Dated October 30, 1973 lnventofls) Hans Olof Samuelson et a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Pag e l 75] delete "both" {73] "Och" should be och Column 7 line 63 "succini" should be succinic Column 9, line 18 "being" should be bring Column 12, lines 60-65: HOC H I HOC2H4--N on so HOC2H4 CH3 should be noc n HOC H N ch so HCrC H CH Column 14, line 53 "dirst" should be first Column 16 line 36 "-35" should be -45 Column 17 line 63 is high yield" should be in high yield Signed and Scaled this Twenty-eighth D3) Of September 1976 [SEAL] A nest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner nflalenrs and Trademarks UNITED STATES PATENT OFFICE s/a-a) CERTIFICATE OFCORRECTION Patent No. 3,769,152 Dated October 30, 1973 Inventor) Hans Olof Samuelson et a1.

It is certified that error appears in the above-identified patent and that: said Letters Patent are hereby corrected as shown below:

age 1 [75] t delete "both" T j ['73] "Och" should be och Column '7 line 63 "succini" should be succinic Column 9 line 18 "being" should be bring Column 12, lines 60-65: 1100 3;

HOC H --N CH SO" HOC H CHJ should be HOC H v HOC H N cu s o HOC H CH Column 14, line 53 "dirst" should be first 9 Column 16 line 36 "-35" should be -45 7 Column 17 line 63 is high yield" should be in high yield Signed and Sealed this Twenty-eighth Day Of September 1976 [SEAL] Altest:

RUTH c. msou c. MARSHALL DANN Arresting Office (umml'ssl'nmr uj'lalenls and Trazlemurkx 

2. The process of claim 1, in which the amount of alkali is within the range from about 5 to about 20 percent of the total molar quantity of alkali required.
 3. The process of claim 1, in which the alkali is added continuously during the digestion.
 4. The process of claim 1, in which the aLkali is added in increments during the digestion.
 5. The process of claim 1, in which the pH is within the range from about 9.5 to about 11.5.
 6. The process of claim 1, in which pH is permitted to drop below 9, down to about 8, during the final stages of the process without seriously affecting the quality of the pulp.
 7. The process of claim 1 in which the total amount of alkali is within the range from about 1 to about 10 kilomoles per 1,000 kg. of dry wood.
 8. The process of claim 7, in which the cellulose pulp produced is a viscose, acetate or cuprammonium pulp, and the amount of alkali is within the range from about 6 to about 8 kilomoles per 1,000 kg. of dry wood.
 9. The process of claim 7 in which the cellulose pulp produced is a semichemical pulp, and the amount of alkali is within the range from about 1 to about 2 kilomoles per 1,000 kg. of dry wood.
 10. The process of claim 7, in which the cellulose pulp produced is a paper pulp, and the amount of alkali is within the range from about 2.5 to about 5 kilomoles per 1,000 kg. of dry wood.
 11. The process of claim 7, in which the cellulose pulp produced is a pulp for use in making fine paper, plastic fillers, and soft paper or tissue paper, and the amount of alkali is within the range from about 2 to about 6 kilomoles per 1,000 kg. of dry wood.
 12. The process of claim 1 in which the alkali is any alkali metal hydroxide or alkali metal carbonate.
 13. The process of claim 12 in which the alkali metal hydroxide is sodium hydroxide.
 14. The process of claim 12 in which the alkali metal carbonate is sodium carbonate.
 15. The process of claim 1 in which the alkali is a mixture of alkali metal hydroxide or alkali metal carbonate with an alkali metal bicarbonate which serves as a buffer.
 16. The process of claim 15 in which the alkali metal bicarbonate is sodium bicarbonate.
 17. The process of claim 16 in which the amount of alkali metal bicarbonate is within the range from about 1 to about 5 kilomoles per 1,000 kg of dry wood.
 18. The process of claim 1 in which the aqueous alkali includes a buffering agent which increases the buffer capacity of the digestion liquor, and assists in maintaining the pH within the prescribed range during the digestion.
 19. The process of claim 18, in which the pH is maintained within the range from about 9.5 to about 9.7.
 20. The process of claim 18, in which the buffering agent is a waste liquor selected from the group consisting of waste liquor from a previous alkaline oxygen digestion and waste liquor from an alkaline oxygen bleaching process.
 21. The process of claim 1, in which the initial alkali charged comprises an alkali metal carbonate and the remainder of the alkali charged as the digestion proceeds is an alkali metal hydroxide.
 22. The process of claim 21 in which the initial alkali charged also includes an alkali metal bicarbonate.
 23. The process of claim 1, in which the alkali comprises a mixture of an alkali metal hydroxide and alkali metal carbonate.
 24. The process of claim 1, in which the digestion temperature is within the range from about 90* to about 175*C.
 25. The process of claim 1, in which, during the major portion of the digestion, the partial pressure of oxygen is within the range from about 3 to about 12 atmospheres.
 26. The process of claim 1, in which the aqueous alkali comprises an inhibitor inhibiting attack on the hemicellulose and cellulose.
 27. The process of claim 26, in which the inhibitor is a magnesium compound.
 28. The process of claim 27, in which the magnesium compound is a magnesium complex selected from the group consisting of an inorganic or organic acid complex of polyhydroxy acids, organic acids containing at least two carboxylic acid groups, and polyphosphoric acids.
 29. The process of claim 28, in which the organic acid is an aliphatic Alpha - or Beta -hydroxy carboxylic acid.
 30. The process of claim 26, in which the inhibitor is an alkali-soluble silicic acid or alkali metal silicate.
 31. The process of claim 1, in which the wood is pretreated with water or an aqueous acidic, neutral or alkaline solution before the digestion.
 32. The process of claim 31, in which the aqueous solution comprises a sulfite precursor selected from sulphur dioxide, an alkali metal bisulphite, or an alkali metal sulphite.
 33. The process of claim 31, in which the pretreatment is carried out in the presence of a complexing agent for bivalent and or polyvalent ions of transition metals.
 34. The process of claim 1, in which a surface-active substance is present during at least a part of the digestion process.
 35. The process of claim 1, in which the wood is a hardwood selected from birch, beech, poplar, aspen, maple, alder and eucalyptus.
 36. The process of claim 1, in which the wood is in the form of particles having an average thickness of at most 3 mm. 